Blowfish, also called puffer fish, are members of the order Tetraodontiformes. They’re known for their spiky skin structures, which can cover almost the entire body or leave parts of it bare.
Researchers have now figured out how these spines, which are actually modified scales, develop during development. Their findings show that some seemingly radical evolutionary innovations may simply be a matter of minor tinkering.
Spines
The spines of puffer fish are a defense mechanism that prevent predators from swallowing the bloated creature, which contains poisonous tetrodotoxin. These sharp, needle-like appendages are visible when the fish inflates, which happens if a predator approaches it or when two male pufferfish compete for a female’s attention during mating season. When the fish deflates, the spines are hidden in the skin.
Pufferfish, like all tetraodontiforms (including basal spikefishes and three-toothed pufferfish), display a rich diversity of dermal ornamentations, including spines, plates, and scales. However, the developmental basis for this diversity is unclear.
To address this question, researchers analyzed the gene expression patterns of developing scales, spines, and plates in Japanese grass pufferfish Takifugu niphobles using a genomic approach. They found that the same set of genes that shape the development of zebrafish scales, mouse hair, and chicken feathers are also involved in shaping pufferfish spines.
Specifically, the team discovered that a gene called fst regulates the patterning of spinoid scales and spines, and when fst is blocked, both scales and spines are reduced in number and morphology. This suggests that changes in fst expression during various stages of appendage development have contributed to the tremendous diversity of spineoid plate and spine morphology seen in pufferfishes and other tetraodontiforms.
Fraser and colleagues next tested whether the same genes that control the formation of scales and spines also affect how they develop in embryos. They used CRISPR and other editing techniques to block particular genes that are classic markers of skin appendage development. This allowed them to reduce the number of spines that appeared on pufferfish and loosen the restriction on where the spines grew. Normally, these spines are localized to specific areas of the fish that offer the most protection.
Pufferfish are not the only animals with unusually shaped spines and other body features, but it’s the first time that scientists have investigated the link between these traits and specific genes. “It just blows me away that, no matter how evolutionarily different a skin structure may be from another, it still depends on the same collection of genes,” says Fraser.
Ribs
The puffer fish’s ribless skeleton allows it to expand into a tight sphere. When threatened, the fish gulps water quickly and swells into a hard-shelled ball of three times its normal size. It also puffs out spiky spines to make itself even less palatable for predators. This behavior is so unique that biologists have struggled to figure out how it evolved in the first place.
Fossils haven’t revealed much, but researchers have found clues from the genetic code. Pufferfish share a reduced genome with ray-fin fish, including some sharks and squid. The genes that code for Hox clusters are conserved in these fish. However, a particular gene, Hoxa7a, has become a pseudogene in pufferfish Spheroides nephelus and Japanese pufferfish Takifugu rubripes (fugu). Hoxa7a is still intact in striped bass (Genus Sauridae), which is more closely related to pufferfish than either medaka or zebrafish. Thus, the loss of Hoxa7a in pufferfish could be a result of a genetic mutation that happened after the divergence of medaka and zebrafish lineages.
To understand how the puffer fish’s skin becomes puffed, scientists examined embryos of fugu. They found that the fish’s stomach stretches out into the peritoneal space, filling gaps beneath the head, dorsal fin, anal fin, and caudal peduncle. The stomach also contains accordion-like folds that facilitate expansion. This makes it possible for a pufferfish to balloon up to three times its normal size in a matter of seconds.
A pufferfish that’s inflated releases a poison called tetrodotoxin, which is deadly to fish but has no effect on humans. It also depresses the heart rate, so a predator that manages to snag one will not live for very long.
Pufferfish also eat invertebrates, mainly invertebrates like snails, crabs, and shrimp. The large specimens are capable of cracking open the hard shells of clams and mussels to get at the flesh inside. They also consume algae, which provide important nutrients. The pufferfish is a slow swimmer, so it has to rely on its ability to swell up as a defense mechanism against predators. If a predator does manage to snag one, the fish has another way to escape: it can explode.
Pelvic Fins
Pufferfish and their relatives are among the most diverse fishes in the world, with a wide variety of shapes, sizes, colors, and behaviors. They range from tiny, spherical critters that can fit in the palm of your hand to large predators that may exceed 10 feet (3 meters) in length and weigh more than 6,700 pounds (2,721 kilograms). While their scales are often drab or brown, some species display beautiful patterns of color, while others have sharp spines and other antipredator adaptations. Pufferfish are capable of rapidly inflating their stomachs, which can increase in size by 50-100-fold, depending on the species. This ability allows them to escape from predators or to frighten potential prey by appearing as a hard, spherical ball. Inflated puffers also possess pointed spines and can expel a poisonous gas called tetrodotoxin, which can incapacitate or kill larger predators by blocking their gills and interfering with breathing.
Unlike most chordates, which have a pleural fin fold that passes posteriorly to the anus and connects to the ventral portion of the tail fin, pufferfish lack this structure. Scientists have theorized that this secondarily simplified body plan is the result of mutations that have reduced complexity of Hox cluster genes.
To determine whether Hox gene changes were responsible for pelvic fin loss in pufferfish, scientists isolated the genomes of the southern pufferfish Spheroides nephelus and of the Japanese pufferfish Takifugu rubripes (fugu). They compared the expression of genes related to appendage positioning and initiation during fugu development. During the positioning phase, they found that both pufferfish and anadromous sticklebacks maintained Hoxd9 transcripts in the region of the pectoral fin bud, but fugu did not develop a pelvic fin. Similarly, during the outgrowth phase, the fugu genome showed no evidence of Hoxd9 transcription in the pelvic bud region.
Alcian blue staining of fugu embryos and larvae revealed no cartilaginous elements in the pelvic region. This is in contrast to the pattern seen in tetraodontids, which have a pelvic fin, and in the extinct diodontid Eotetraodon. These results support the hypothesis that the loss of a pelvic fin in pufferfish evolved via a series of genetic mutations that blocked earlier stages of the appendage formation program.
Bones
The bones of a puffer fish are not like those of a typical fish. They lack ribs and pelvic bones, and have fused bones in the cranium and jaw. This makes the bones much lighter and more flexible than those of other teleost fishes. The skeletal system is supported by tendons that connect muscles to skeletal structures, and this gives them tremendous flexibility. Puffer fishes have very short limbs and use a special kind of fin to help them move about.
They are able to change their shape and size by swallowing water or air. This allows them to inflate into a round, nearly inedible ball several times their original size. They can also release an extremely toxic substance, called tetrodotoxin, that renders the fish foul tasting and deadly to predators.
When a pufferfish is not inflated, the spines are hidden under spiky dermal armor. However, when the fish inflates, the spiky bones come out of hiding to form a rigid armor that looks very cool. The spiky bones are not actually connected, and they layer together like caltrops to make a very stiff armor of spikes that is only visible when the fish inflates.
Pufferfish do not have ribs because the skeleton would only get in the way of their ability to expand. They have a few vertebrae and no pelvic bones because, again, they needed to be able to expand. They also have no sternal or costal bones, and their jaws are supported by a single bone in the snout.
The pufferfish is one of the most enigmatic animals in the world, and their ability to puff up is incredible. Pufferfish also have a very strong digestive tract, and they are known to poison people and pets with their highly toxic tetrodotoxin.
Researchers are studying how pufferfish evolved in order to understand how they can grow and shrink so dramatically. They are examining the genes that control their Hox clusters, which are regions of DNA responsible for the development of many different body parts and behaviors. They are comparing the genomes of the southern pufferfish Spheroides nephelus with that of the Japanese fugu Takifugu rubripes to see if these two species share a similar Hox cluster evolution. They are also analyzing the Hox clusters of zebrafish to learn more about genome duplication.
